a) Field of the Invention
This invention relates to a powder coating, a powder coating process and a powder-coated article, and more specifically to a powder coating useful in an electrostatic coating process making use of a powder-fluidizing container, a powder coating process for the powder coating and a powder-coated article obtained by using the powder coating.
b) Description of the Related Art
Processes for coating an article with a powder coating conventionally include electrostatic coating processes and fluidized bed coating processes. In an electrostatic coating process, use of a powder coating the average particle size of which is 30 .mu.m or smaller is generally prone to cohesion of particles of the powder coating, resulting in a baked coating of reduced smoothness due to inclusion of cohered particles. Moreover, the flowability of the powder coating as powder is reduced and an object under coating cannot therefore be coated evenly, leading to the formation of a coating with lowered smoothness. An average particle size greater than 30 .mu.m, on the other hand provides a coating of reduced cohesion tendency but, after baking, results in the formation of large interparticle voids. When coated thinly (about 50 .mu.m or less), a coating is obtained with reduced smoothness after baking.
A fluidized-bed coating process, the other coating process, uses a powder coating, particles of which are generally coarser than those employed in an electrostatic coating process so that a coating lower in smoothness than that available by the electrostatic coating process is obtained after baking.
With a view to providing a coating with improved smoothness by relying upon a coating process, a great deal of research has been conducted in recent years on electrostatic fluidized-bed coating processes, resulting in the proposal of such a process as illustrated in FIG. 1.
The process illustrated in FIG. 1 is a powder coating process. Described specifically, a powder coating 2 which has been pneumatically brought into a fluidized state in a powder-fluidizing container 1 is lifted toward an object 4, which is to be coated and is suspended from a ground electrode 7, while applying positive (or negative) charges thereto from a powder-feeding lower electrode 3. Opposite, namely, negative (or positive) charges are next applied from a powder-feeding upper electrode 5, whereby the powder coating is lifted further while its charges are being neutralized. A pair or plural pairs of side electrodes 6 are arranged above the powder-feeding upper electrode 5 so that the object 4 suspended from the ground electrode 7 is conveyed between the paired side electrodes 6 in parallel with the paired side electrodes 6. Negative (or positive) charges are then applied from the pair or plural pairs of side electrodes 6, whereby powder coating is applied to a surface of the object passing between the side electrodes 6.
According to this process, the powder coating 2 is fluidized in the powder-fluidizing container 1. Charges are applied to the cohered powder coating 2 from the lower electrode 3. Individual particles of the powder coating are hence charged so that they repel each other. The cohered particles are therefore lifted while separating from each other. They are lifted further while their charges are once neutralized by the upper electrode 5. Subsequently, charges are applied from the side electrodes 6 so that the powder coating is applied.
The above-described electrostatic coating process making use of the powder-fluidizing container (electrostatic fluidized-bed coating process) has an excellent advantage that a thinner coating can be formed compared with conventional power coating processes. This process is however accompanied by a problem that powder coatings of different colors or different resins have to be prepared in advance, in other words, an extremely wide variety of powder coatings have to be prepared beforehand because only one kind of powder coating is used upon each application.